1. Field of the Invention
The present invention relates to optical fibers for use in a light amplifier, and more particularly, to an optical fiber for used in a light amplifier having an improved optical amplification efficiency.
2. Description of the Related Art
In general, in the manufacture of light amplifiers having a wavelength in the region of 1310 nm which is the zero dispersion wavelength of silica glass, neodymium ions (Nd.sup.+3) and praseodymium ions (Pr.sup.+3), which are used as rare earth elements, are implanted into the glass. However, using Nd.sup.+3 causes the following problems.
In particular, the peak wavelength of fluorescence produced in the .sup.4 F.sub.3/2.fwdarw..sup.4 I.sub.13/2 transition, which occurs at a wavelength of 1.35 .mu.m, is considerably removed from the zero dispersion wavelength. The intensity of fluorescence in the wavelength region of 1.3 .mu.m is very weak compared to that of fluorescence emitted at other wavelength regions of 890 nm and 1064 nm in the .sup.4 F.sub.3/2 level. Also, the optical gain at wavelengths lower than 1.3 .mu.m is significantly lowered, due to excited state absorption (ESA) at the .sup.4 F.sub.3/2 level.
In order to solve these problems, a method of using a fluoride-rich glass instead of silica glass as a base material has been suggested. However, this method does not achieve a great improvement in optical gain in the wavelength region of 1310 nm.
Meanwhile, in the case of using Pr.sup.+3, which is a rare earth element for use in doping glass, fluorescence emitted during the .sup.1 G.sub.4.fwdarw..sup.3 H.sub.5 transition can be used. Also, the transition probability at such levels is very high compared that at other levels, so that a high optical amplification efficiency is expected at the transition from .sup.1 G.sub.4 to .sup.3 H.sub.5 levels.
However, in the actual application of Pr.sup.+3, the energy gap between the .sup.1 G.sub.4 level and the .sup.3 F.sub.4 level, which is immediately below the .sup.1 G.sub.4 level, is as small as 3000 cm.sup.-1. Thus, in the case of using an oxide glass having a high lattice vibration energy (&gt;800 cm.sup.-1), the probability of non-radiative transition of Pr.sup.+3 excited to the .sup.1 G.sub.4 level due to multiple lattice vibration relaxation is greatly increased, thus lowering optical amplification efficiency.
To solve this problem, U.S. Pat. No. 5,379,149, to Snitzer et al., entitled GLASS COMPOSITIONS HAVING LOW ENERGY PHONON SPECTRA AND LIGHT SOURCES FABRICATED THEREFROM, discloses a method of using a germanium-galium-sulfur (Ge--Ga--S) glass as a base material. The patent in particular describes sulfur rich germanium-gallium-sulfur compositions doped with rare earth ions. Here, the lattice vibration energy of the Ge--Ga--S glass is smaller than that of a conventional oxide glass or fluoride glass, so that an improvement in optical amplification efficiency at the wavelength region of 1310 is anticipated.
K. Wei, Ph.D. dissertation, "Synthesis and Characterization of Rare Earth Doped Chalcogenide Glasses" Rutgers University (1994), pp. 71-79, also describes Ge--Ga--S glasses with excess sulfur relative to the stoichiometric composition line of Ge--S.sub.2 --Ga.sub.3 S.sub.3. Only sulfur rich compositions were studied because sulfur poor compositions were stated to be usually opaque. K. Wei has reported that when a Ge--S, As--S, or Ge--As(P,Sb)--S glass has a stoichiometric composition or contains more sulfur than in the stoichiometric composition, the degree of solubility of rare earth element with respect to such glass is no more than several hundreds of parts per million (ppm). Thus, in the case where excessive rare earth element is added, fine crystals are precipitated due to the clustering of rare earth ions, causing devitrification.
The described Ge--Ga--S glass compositions contain excessive sulfur (S) compared to the stoichiometric composition line of GeS.sub.2 --Ga.sub.3 S.sub.3, as shown in a phase diagram of Ge--Ga--S. Also, in the case of using Ge.sub.25 Ga.sub.5 S.sub.70 glass which is a typical example of the composition, the solubility of Pr.sup.+3 is relatively high compared to a Ge--S, As--S, or Ge--As(P,Sb)--S glass which has been adopted by a conventional method. However, if a high concentration of Pr.sup.+3 is added, the energy transfer rate between ions is markedly increased due to clustering of Pr.sup.+3 ions, so that the fluorescence lifetime at the .sup.1 G.sub.4 level is shortened, in addition to lowering the optical amplification efficiency.